Directional signal and noise sensors for borehole electromagnetic telemetry system

ABSTRACT

An electromagnetic borehole telemetry system providing improved signal to noise ratio. Adaptive filters use noise channels as references to remove noise from the signal channel. Directional detectors provide a signal channel with reduced noise content and improved noise channels with reduced signal content. Directional detectors may be magnetometers aligned with the magnetic field, or antennas aligned with the electric field, of signal or noise sources. Alignment may be performed by beam steering of the outputs of a three-channel detector, which may detect both signal and noise.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to U.S. patent application Ser. No.______ [attorney docket 1391-25201] entitled “Motion Sensor for NoiseCancellation in Borehole Electromagnetic Telemetry System”, filed on thesame date as this application by the present inventors and assigned tothe same assignee, which is hereby incorporated by reference for allpurposes.

[0002] This application is related to U.S. patent application Ser. No.______ [attorney docket 1391-25202] entitled “Filters for CancelingMultiple Noise Sources in Borehole Electromagnetic Telemetry System”,filed on the same date as this application by the present inventors andassigned to the same assignee, which is hereby incorporated by referencefor all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0003] Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

[0004] Not applicable.

FIELD OF THE INVENTION

[0005] This invention relates to a borehole electromagnetic telemetrysystem, and in particular to a system for increasing the signal to noiseratio of wellbore electromagnetic telemetry signals includingdirectional detectors for enhancing telemetry signal and noise referencechannels.

BACKGROUND OF THE INVENTION

[0006] Without limiting the scope of the invention, its background isdescribed in connection with transmitting downhole data to the surfaceduring measurements while drilling (MWD), as an example. It should benoted that the principles of the present invention are applicable notonly during drilling, but throughout the life of a wellbore including,but not limited to, during logging, testing, completing and production.The principles are also applicable to transmission of signals from thesurface to downhole equipment.

[0007] Heretofore, in this field, a variety of communication andtransmission techniques have been attempted to provide real time datafrom the vicinity of the bit to the surface during drilling. Theutilization of MWD with real time data transmission provides substantialbenefits during a drilling operation. For example, continuous monitoringof downhole conditions allows for an immediate response to potentialwell control problems and improves mud programs.

[0008] Measurement of parameters such as bit weight, torque, wear andbearing condition in real time provides for more efficient drillingoperations. In fact, faster penetration rates, better trip planning,reduced equipment failures, fewer delays for directional surveys, andthe elimination of a need to interrupt drilling for abnormal pressuredetection is achievable using MWD techniques.

[0009] At present, there are four major categories of telemetry systemsthat have been used in an attempt to provide real time data from thevicinity of the drill bit to the surface; namely, mud pressure pulses,insulated conductors, acoustics and electromagnetic waves.

[0010] In a mud pressure pulse system, the resistance of mud flowthrough a drill string is modulated by means of a valve and controlmechanism mounted in a special drill collar near the bit. This type ofsystem typically transmits at 1 bit per second as the pressure pulsetravels up the mud column at or near the velocity of sound in the mud.It is well known that mud pulse systems are intrinsically limited to afew bits per second due to attenuation and spreading of pulses.

[0011] Insulated conductors, or hard wire connection from the bit to thesurface, is an alternative method for establishing downholecommunications. This type of system is capable of a high data rate andtwo way communication is possible. It has been found, however, that thistype of system requires a special drill pipe and special tool jointconnectors which substantially increase the cost of a drillingoperation. Also, these systems are prone to failure as a result of theabrasive conditions of the mud system and the wear caused by therotation of the drill string.

[0012] Acoustic systems have provided a third alternative. Typically, anacoustic signal is generated near the bit and is transmitted through thedrill pipe, mud column or the earth. It has been found, however, thatthe very low intensity of the signal which can be generated downhole,along with the acoustic noise generated by the drilling system, makessignal detection difficult. Reflective and refractive interferenceresulting from changing diameters and thread makeup at the tool jointscompounds the signal attenuation problem for drill pipe transmission.

[0013] The fourth technique used to telemeter downhole data to thesurface uses the transmission of electromagnetic waves through theearth. A current carrying downhole data signal is input to a toroid orcollar positioned adjacent to the drill bit or input directly to thedrill string. When a toroid is utilized, a primary winding, carrying thedata for transmission, is wrapped around the toroid and a secondary isformed by the drill pipe. A receiver is connected to the ground at thesurface where the electromagnetic data is picked up and recorded. It hasbeen found, however, that in deep or noisy well applications,conventional electromagnetic systems are unable to generate a signalwith sufficient intensity to be recovered at the surface.

[0014] In general, the quality of an electromagnetic signal reaching thesurface is measured in terms of signal to noise ratio. As the ratiodrops, it becomes more difficult to recover or reconstruct the signal.While increasing the power of the transmitted signal is an obvious wayof increasing the signal to noise ratio, this approach is limited bybatteries suitable for the purpose and the desire to extend the timebetween battery replacements. It is also known to pass band filterreceived signals to remove noise out of the frequency band of the signaltransmitter. These approaches have allowed development of commercialborehole electromagnetic telemetry systems which work at data rates ofup to four bits per second and at depths of up to 4000 feet withoutrepeaters in MWD applications. It would be desirable to transmit signalsfrom deeper wells and with much higher data rates which will be requiredfor logging while drilling, LWD, systems.

SUMMARY OF THE INVENTION

[0015] The present invention provides apparatus which improves thesignal to noise ratio in an electromagnetic telemetry system whichtelemeters data between a borehole and the surface of the earth. Areceiver includes a noise canceller which uses a reference noise channelto remove noise from a received signal. The present invention includessensors which provide a signal channel with improved signal to noiseratio and sensors which provide one or more noise channels which haveimproved noise to signal ratio. An improved sensor includes adirectional magnetic field detector, e.g. a magnetometer, or adirectional electric field detector, e.g. an antenna, positioned and/oraligned to preferentially receive signal or noise. Alignment ispreferably by using a three-axis detector and beam steering apparatusfor aligning the detector with the signal source for the signal channelor with a noise source for a noise channel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is an illustration of an oil well drilling rig and awellbore electromagnetic telemetry system in use while a well is beingdrilled.

[0017]FIG. 2 is a block diagram of an adaptive filter used to removenoise from a received electromagnetic signal.

[0018]FIG. 3 is a more detailed block diagram of the filter of FIG. 2and a model of signal and noise transmission paths.

[0019]FIG. 4 is a block diagram illustrating the structure of anadaptive transversal filter.

[0020]FIG. 5 is a block diagram illustrating a filter tap coefficientalgorithm for the filter of FIG. 4.

[0021]FIG. 6 is a block diagram of a three-axis magnetometer andapparatus for beam steering the magnetometer to alignment for optimalreception of electromagnetic radiation from a noise source.

[0022]FIG. 7 is a block diagram of a three-axis magnetometer andapparatus for beam steering the magnetometer to alignment for optimalreception of a telemetry signal generated downhole.

[0023]FIG. 8 is a block diagram of a system for combining multiple noisechannels and removing the combined noise from a signal channel with anadaptive filter.

[0024]FIG. 9 is a block diagram of a system for removing multiple noisesources from a signal channel by use of multiple adaptive filters inseries.

DETAILED DESCRIPTION OF THE INVENTION

[0025] With reference to FIG. 1, a wellbore electromagnetic, EM,telemetry system will be described. A drill rig 10 is shown driving adrill pipe 12 in a wellbore 14. The drill pipe 12 has a drill bit 16 onits lower end. A motor 18 on the rig 10 represents an electric motorwhich may rotate the drill pipe 12 and also represents other motorswhich would be used, e.g. to pump mud through the drill pipe 12. Thedrilling mud may be used to drive a mud motor located just above drillbit 16.

[0026] An electronics package 20 is positioned within the drill pipe 12near drill bit 16. The electronics package includes sensors formeasuring parameters, such as pressure and temperature, and atransmitter for telemetering the information to the surface location ofthe well. Any means of transmitting an electromagnetic signal may beused. In this figure, the package 20 is shown driving an electriccurrent into two sections of drill pipe 12 separated by an insulatingsection 22. Alternatively a toroidal core may be positioned around drillpipe 12 and the package 20 may drive a winding on the core to generatean equivalent transmitted electromagnetic signal. The toroidal core isusually an integral part of a section of drill pipe or a drill collar toprotect the core from damage.

[0027] A transmitted electromagnetic signal is represented by lines ofcurrent 24 and equipotential lines 26. This signal is detected at thesurface location of the well and coupled to a signal processor 28. Thesignal may be detected in several ways. Electrical connections 30 may bemade between a surface casing 32 and an electrode 34 implanted into thesurface of the earth some distance from the casing 32. The electricfield (E field) of the transmitted signal produces a voltage betweencasing 32 and electrode 34. This E field detector may be considered adirectional antenna which detects a horizontal component of a potentialdifference arising from the electric field of an EM signal. Thispotential difference may be amplified by an amplifier 36 and thencoupled to signal processor 28.

[0028] The magnetic field component of the transmitted EM signal 24, 26may also be detected. A magnetometer 38 may be positioned in a locationselected to receive the transmitted EM signal. The detected magneticfield may be coupled to amplifier 36 and used as the signal channel, ormay be combined with the electrical signal from lines 30.

[0029] In the preferred embodiments, a plurality of sensors are used todetect various noise sources which generate EM noise. There are a numberof sources of EM noise which is also detected by the signal sensors,e.g. sensor 38, and which therefore reduces the signal to noise ratio ofthe signal channel coupled to signal processor 28. The outputs from thevarious noise sensors are referred to herein as noise channels. It ispreferred that the noise channels contain none of the signal transmittedfrom the transmitter package 20.

[0030] A plurality of noise sensors 40 may be positioned at variousdistances from the drill rig 10. Physical spacing tends to reduce theamount of transmitted signal detected by sensors 40. The sensors 40 maybe positioned near sources of noise such as power lines, motors,generators, and pipelines to more effectively detect noise from suchsources. At least one sensor may be placed away from such manmade noisesources to detect magnetotelluric noise. In a preferred embodiment, thesensors 40 are magnetometers or include a magnetometer and an electricalfield or current detector. In a more preferred embodiment, the sensors40 include three-axis magnetometers and beam steering means, asdescribed in detail below. By proper selection of sensor type and byproper positioning, physically or by beam steering, the sensors canprovide a noise channel with minimum signal.

[0031] One or more sensors 42 may be mounted on the drill rig 10 todetect noise. These sensors may include current detectors for detectingdrive currents in motors such as motor 18 or output currents ofelectrical generators which provide current to the motors. The sensors42 may preferably include magnetometers as discussed above. In oneembodiment, sensors 42 may include motion sensors, e.g. seismometers,which detect physical motion, e.g. vibration, in various parts of thedrill rig 10 and equipment which drives the drill pipe 12. The sensors42 may be attached to structural members of rig 10 or placed on floormembers 11. In addition, sensors 42 may be coupled to the earth nearsupport members of rig 10 to detect earth motion induced by the rig 10.

[0032] As discussed above, the signal processor 28 receives a signalchannel from amplifier 36 and also receives one or more noise channelsfrom various noise detectors 40 and 42. As discussed in the backgroundsection above, the processor 28 may include bandpass filters on allchannels which block all signals outside the frequency band in which theEM transmitter 20 operates. In addition, the processor 28 includes oneor more noise cancellers which, by reference to the noise channels,remove noise from the signal channel.

[0033] With reference to FIG. 2, a preferred noise canceller 44 will bedescribed. A preferred noise canceller 44 includes an adaptivetransversal filter 46. The canceller 44 has two inputs, a primary, i.e.signal channel, input 48 and a reference, i.e. noise channel, input 50.The adaptive filter 46 has a reference input forming the reference input50 of the noise canceller 44 and has an output 52 providing anapproximation of the noise contained in the signal on canceller 44 input48. The canceller 44 also includes an adder 54 which removes theestimated noise on filter 46 output 52 from the primary input on line 48to form an error signal e(t) on line 56. The error signal is fed back tofilter 46 and also forms the output of canceller 44 which is a noisefree signal, or at least an approximation thereof, having improvedsignal to noise ratio.

[0034] For best results, the noises in the primary input 48 and in thereference input 50 must be correlated and the reference input should befree of the signal. The object is to use the reference input to reducethe noise in the primary input. To the extent that the noise channelincludes desired signal, the canceller will cancel part of the signal.

[0035] For the purpose of illustration, assume that the primary input 48is the up link electric field signal received by an EM telemetry systemthrough leads 30 (FIG. 1) and that this signal has been corrupted bynoise induced by rotation of drill pipe 12. This signal therefore hasthe following form:

f(t)=s(t)+n 1(t)

[0036] where the received signal, f(t), is the sum of the electric fieldcomponent of the telemetry signal, s(t), plus the electric field noisecomponent, n1(t), induced, for example, by drillstring 12 rotation. Theprimary signal may be sampled at regular intervals, T, and digitized toproduce the following discrete-time signal:

f _(i) =s _(i) +n 1 _(i),

[0037] where i refers to the sample number, from an arbitrary timeorigin, common to all measurements.

[0038] The reference input may be expressed in discrete time as:

y _(i) =n 2 _(i)

[0039] Where n2 _(i) is the reference noise signal which is assumed tobe correlated with the primary noise signal, n1(t), corrupting thetelemetry signal. This noise reference can be obtained using amagnetometer or an electric field sensor at a point sufficiently removedfrom the location where the primary signal is received so that there isno appreciable component of the telemetry signal in it. The correlationbetween n2 and n1 can be exploited to minimize the noise in the primaryinput. In general, the exact nature of this correlation need not beknown in advance for this noise cancellation method to work.

[0040] With this notation, noise cancellation is seen to be simply thejoint process estimation problem whose structure is shown in FIG. 2. Theadaptive joint process estimation algorithm will be able to exploit thecorrelation between the two input signals to minimize the mean-squareerror, E[e(t)²], between f(t) and an estimator of the noise, n3(t),where:

e(t)=f(t)−n 3(t)

[0041] or in discrete form,

e _(i) =f _(i) −n 3 _(i)

[0042] Taking into account the assumption that n2(t), and hence n3(t),are uncorrelated with s(t),

E[e(t)]=E[s(t)]+E[(n 1(t)−n 3(t))]

[0043] or

E[e _(i) ]=E[s _(i) ]+E[(n 1 _(i) −n 3 _(i))]

[0044] where E[ ] denotes expected value of the quantity in brackets []. Adjusting the adaptive filter such that the mean squared value ofE[e_(i)] is minimum results in n3(t) being the best estimator of n1(t).

[0045] In its simplest embodiment, this invention uses an adaptivefilter to approximate the transfer function between a referenceelectromagnetic noise signal picked up by a magnetometer, e.g. sensor 40of FIG. 1, and electromagnetic noise contaminating the telemetry signalby minimizing the mean-squared error between them. The telemetry noiseapproximation derived from an adaptive filter is subtracted from thenoisy telemetry signal to get a “noise-free” telemetry signal, or atleast a signal with improved signal to noise ratio.

[0046]FIG. 3 provides a more detailed block diagram of an EM telemetrysystem and a model of signal and noise channels. Original data, d(t), isdigitized, encoded, modulated and then radiated as a telemetry signalinto the earth-pipe electromagnetic transmission channel by theelectromagnetic transmitter (E/M XMTR), 58, e.g. part of the electronicspackage 20 of FIG. 1. The electromagnetic telemetry signal istransmitted uphole via the earth-pipe transmission channel where it ispicked up as a difference signal between the borehole casing 32 at thesurface and earth electrode 34 (FIG. 1). The earth-pipe-electrodetransmission channel 60 is represented as a transfer function G2(s)which results in a signal s(t) being received at the surface location.The telemetry signal detected by the electrodes at the surface iscontaminated by electromagnetic noise sources near the surface such asmachinery (primarily on the drilling rig) and power lines. The transferpath 62 between the reference noise source, n2(t), and the telemetrynoise, n1(t), is denoted as transfer function G1(s) which results innoise n1(t) reaching the signal detector. In FIG. 3, an adder 64 is usedto model the combination of the transmitted data d(t) and the EM noisesource n2(t) to form the signal channel 66, s(t)+n1(t), which is theprimary input to the noise canceller. The combination actually occursbecause the signal sensor, e.g. the voltage detected between casing 32and electrode 34, detects both the signal, s(t), and noise, n1(t).

[0047] The output 68 of a magnetometer 70 forms the noise channel orreference input, n2(t), into a noise canceller 72 including an adaptivefilter 74. Both the signal channel 66 and noise channel 68 may beconverted to digital form by analog to digital converters 76. Theadaptive filter 74 transforms the reference noise signal, n2(t), into anapproximation n3(t) of the telemetry noise n1(t) at its output 78. Thedifference between the filter's output 78 and the noisy telemetry signal66 is produced by subtractor 80 and is used as the error signal, e(t),into the adaptive filter input 82, which also forms the output of thecanceller 72. The adaptive filter minimizes the error signal byadjusting its output to be as close an approximation (in the mean squaresense) to the noisy telemetry signal as possible. Since the referencenoise input, n2(t), is a function of the telemetry noise, n1(t), but nota function of the telemetry signal, s(t), and since the signal and noiseare not correlated, the filter can only force the reference toapproximate the telemetry noise, but not the telemetry signal. Theresult of the process is that the error signal, e(t), is anapproximation of a noise free signal s(t). This improved signal, i.e.the approximation of a noise free signal s(t), is coupled to a receivermodule 84 for further processing to reconstruct the original transmitteddata d(t). If the signals into noise canceller 72 were converted fromanalog to digital form by converters 76, a digital to analog converter86 may be used to convert the output 82 of canceller 72 back to analogform for receiver 84.

[0048]FIG. 4 provides a schematic of the adaptive filter 74 of FIG. 3.The digitized input signal 88 (e.g. a noise channel from a magnetometer)is run through a series of unit time delays 90 of T seconds, eachdesignated as Z⁻¹. The signals are “tapped off” after unit time delayand each multiplied in multipliers 92 by unique filter tap coefficientsC₁, C₂, . . . , C_(n). The output of the filter is formed by summingtogether the gain-adjusted tap signals at the outputs of multipliers 92in summer 94. The filter's transfer function is determined by the valueof the filter's tap coefficients. The filter's transfer function isadapted by changing the values of the filter tap coefficients C₁, C₂, .. . , C_(n).

[0049]FIG. 5 shows the filter's adaptation algorithm for one of thefilter's coefficients. The tap coefficients are updated after every“shift” (every T seconds) of the digitized reference signal through thefilter's tapped delay line. The coefficient at the j^(th) tap is updatedby a value equal to the respective tap signal, y(T−j ) times thecanceller's digitized error signal, e(T), times a small adaptationcoefficient, β. The adaptation algorithm may be represented by thefollowing equation:

cj _(i+1) =cj _(i) +β·e _(i) ·y _(i−j)

[0050] For an adaptive filter to work best, the noise reference, ornoise channel, would contain only noise and not contain any of thedesired signal. In real systems, some of the desired signal will bedetected by any EM detector used for detecting a noise source. Priorsystems place noise detectors near noise sources to improve the noisechannel, i.e. increase the noise to signal ratio in the noise channel.In similar fashion, the noise canceller will work better if the signalchannel has as little noise in it as possible, i.e. there will be lessnoise to remove. As with noise detectors, it is known to selectpositions for signal detectors where the maximum signal will be detectedand the minimum noise will be detected. In certain embodiments of thepresent invention, one or more magnetometers are preferred for detectingEM signal and/or noise. A three-axis magnetometer and beam steeringtechniques may be used to provide a noise channel with minimum signalcontent and/or a signal channel with minimum noise content. A three-axismagnetometer is essentially a set of three magnetometers positionedorthogonally to each other with each magnetometer having a separateelectrical output representing the magnetic field in its respectivedirection.

[0051] Both the transmitted EM signal and noise originate as vectorfields. It is possible to receive three different components of eachfield, electric and magnetic, and use these components to fully identifythe vector. In a preferred embodiment, the electric field would bemeasured as shown in FIG. 1. Each of the detectors 40 and 42 wouldinclude a three-axis magnetometer which measures three components of themagnetic field, two components being parallel to the surface of theearth, and the third component being orthogonal to the surface of theearth. Call these three components H_(x), H_(y), and H_(z) respectively.

[0052] As an example of the use of these components, suppose thedownhole EM telemetry transmitter, e.g. package 20 of FIG. 1, is anelectric field type of transmitter and suppose the wellbore is nearlyvertical. Two techniques are commonly employed in the operation ofE-field transmitters. In one of the techniques, an electric current islaunched into the formation and into the drill pipe using a toroidalcoil wound around a section of the drill collar. The other technique isto apply a voltage across an insulating gap. In either case, a currentis launched along the drillstring and into the formation. The componentof the magnetic field of the received signal at the earth's surfacearising from the current launched into the drillstring is parallel tothe earth's surface. This is because the top of a borehole is alwaysorthogonal to the earth's surface, the current flows in the direction ofthe borehole, and the magnetic field due to a current is orthogonal toits direction of flow. When the portion of the well in which thetransmitter is situated is vertical, the magnetic field received at theearth's surface which has propagated through the earth will also tend tobe parallel to the earth's surface. This is because the fieldpropagating through the earth will resemble that due to an electricdipole transmitter with the dipole axis oriented along the boreholeaxis. In this case, the magnetic field is always orthogonal to thedipole axis, and thus parallel to the earth's surface. It is clear inthis case that the in-band, horizontal plane magnetic signals can beused to enhance the telemetry signal picked up using an electric fieldsensor, while the vertical component of the magnetic field, H_(z), canserve as a noise reference, assuming a source of electric field noise iscorrelated with the vertical component of the magnetic field,independent of the signal. A single three-axis magnetometer can be usedin this case. The vertical component of the magnetic field serves as thenoise channel while some linear combination of the electric field andthe horizontal components of the magnetic field serves as the signalchannel.

[0053] If the electric field detector is sufficient for the signalchannel, a single, vertically oriented magnetic receiver can be used forthe noise channel in such an application. A single vertical magnetometertends to not detect the signal because its magnetic field is horizontal,so that its output would be primarily due to noise sources. Thus, whenthe direction of the magnetic field component of an EM signal or noiseis known, or predictable, a single magnetometer physically positioned toprovide a signal channel or a noise channel is a preferred detector. Inonshore, i.e. on land, drilling operations, the electrical component ofthe transmitted EM signal is usually stronger, whereas in offshoredrilling the magnetic component of the transmitted signal is usuallystronger. Therefore, it is preferred to use a magnetometer as a noisedetector onshore and as a signal detector offshore.

[0054] In many cases, the magnetic fields produced by the EM transmitterand various noise sources will not be aligned as discussed above. Thatis, the transmitted signal may produce a magnetic field which is notexactly horizontal, e.g. when drilling deviated holes. Likewise, somesources of electric field noise will produce magnetic field noise havinga predominately horizontal component. This leads to two sensorarrangements, either or both of which may be used in embodiments of thepresent invention. Generally, these two arrangements both use athree-axis magnetometer and beam steering of the three outputs of themagnetometer. In a first case, the beam steering is used to align thedetector with a noise source and, in a second case, it is used to alignwith the signal source. A single three-axis magnetometer may be used forboth purposes simultaneously.

[0055] In the first case, all three outputs of a three-axis magnetometermay respond only to the noise, or at least much more noise than signal,if the magnetometer is sufficiently remote from the telemetry system andthe origin of the noise is not local to the E-field receiver (if it is,the E-field receiver should be moved). The magnetometer outputs can becombined into a single signal which effectively simulates a single-axismagnetometer oriented in the direction of the noise. This output can beused as a noise reference or noise channel, as described earlier.

[0056] In the second case, all three outputs of a three-axismagnetometer may respond primarily to the signal. The magnetometeroutputs can be combined into a single signal which effectively simulatesa single-axis magnetometer oriented in the best direction for receptionof the signal. The electric field signal can be used as a reference forsteering a three-axis magnetometer, and can be further combined with themagnetometer output as an additional signal processing step.

[0057] The procedure of combining the magnetometer outputs to simulate asingle axis magnetometer for these two cases is referred to as “beamsteering.” Specific examples of beam steering a three axis detector toprovide improved noise and signal channels are provided below.

[0058] Beam Steering Magnetometer in Direction of Noise

[0059]FIG. 6 illustrates the apparatus and method used for beam steeringa detector in the direction of EM radiation from a noise source for thefirst case. In this figure the EM telemetry system uses an E fielddetector 96, e.g. casing 32 and electrode 34 of FIG. 1, to detect thesignal transmitted by an EM transmitter, e.g. electronics package 20 ofFIG. 1. A noise channel is provided by a three-axis magnetometer 98comprising magnetometers 100, 102 and 104, positioned orthogonally toeach other. The outputs 106, 108 and 110 of the magnetometers 100, 102and 104 are coupled through filters 112 to multipliers 114 where theyare multiplied by coefficients α, β, and γ. The outputs of themultipliers 114 are combined in adder 116, which provides a noisechannel at its output 118. The output 120 of signal detector 96 is alsocoupled through a filter 112 to the positive input of an adder 122. Thenoise channel 118 is coupled to the negative input of adder 122. Theoutput of adder 122 is the input to an algorithm represented by box 124,which produces the coefficients α, β, and γ which are coupled to themultipliers 114.

[0060] A least squares technique is used to determine threecoefficients, α, β, and γ, such that:

α·H(band_pass_filtered)_(x) +β·H(band_pass_filtered)_(y)+γ·H(band_pass_filtered)_(z) ≈E(band_pass_filtered)

[0061] Given that the magnetic field measurements do not contain thesignal (or contain much more noise than signal), this effectively pointsthe magnetometer toward the noise. To see why this is so, consider anoise source {right arrow over (N)} coming from a specific direction{circumflex over (n)}, where |{circumflex over (n)}|=1, and consider asignal derived from the three magnetometer outputs given by

z=a·H _(x) +b·H _(y) +c·H _(z)

[0062] At what values of a, b, and c will the magnitude of z bemaximized? Note that

[0063] If {circumflex over (n)} is of the form

{circumflex over (n)}=α·î+β·ĵ+γ·{circumflex over (k)}

[0064] then

z=(a·α+b·β+c·γ)·|{right arrow over (N)}|

[0065] or

z={right arrow over (A)}·{circumflex over (n)}·|{right arrow over (N)}|

[0066] where

{right arrow over (A)}=a·î+b·ĵ+c·{circumflex over (k)}

[0067] By one of the basic properties of the inner product, this ismaximized when {right arrow over (A)} is aligned with {circumflex over(n)}.

[0068] The fitting of this combined output to the electric field signalguarantees that the magnetometer is steered toward the common noisesource to which both instruments are responding. That is, at theappropriate values of α, β, and γ, the three-axis magnetometer issynthetically shifted to the direction of the common noise source. Inpreliminary testing of this concept, a simple linear least squares fitof the three magnetometer outputs over about 10 seconds of data wassufficient to determine the coefficients α, β, and γ. This is preferablydone at a time when the downhole transmitter is not operating.

[0069] The bandpass filters 112 of FIG. 6 are optional, but desirable.If they are used, they should be identical. In addition, any signalsampling should be synchronized for all four signals. The output 118 ofthis system can be treated as a noise reference and used with anadaptive noise canceller as discussed above. Alternatively, the threeoutput signals from the magnetometer can first be processed using anadaptive noise canceller, and the resulting three noise estimators canthen be synthetically steered to optimize reception of the noise.

[0070] Beam Steering Magnetometer in Direction of Signal.

[0071]FIG. 7 illustrates apparatus which may be used for steering theoutputs of a three axis detector in the direction of signal. Theapparatus may be identical to the apparatus of FIG. 6 and the samereference numbers are therefore used to identify the various parts. Themain difference between FIG. 6 and FIG. 7 is in the positioning of thethree-axis magnetometer 98. In FIG. 6 the magnetometer 98 is positionedto detect primarily noise, but in FIG. 7, it is positioned to detectprimarily signal in at least two of the magnetometers 100, 102 and 104.As in FIG. 6, the E field detector 96 of FIG. 7 detects the transmittedtelemetry signal.

[0072] Two methods can be employed to effectively steer an outputderived from a three axis magnetometer in the direction of the signal.In the first method, the adder 122 and the algorithm 124 are not needed.Instead, the coefficients α, β, and γ are treated as direction cosinesand calculated based on the anticipated arrival direction of thetransmitted telemetry signal. The arrival direction of the signal doesnot necessarily correspond with the direction from the magnetometerpackage to the signal source, i.e. more than simple geometriccalculations are required. The arrival direction is the direction of themagnetic field lines at the earth's surface arising from the EMtelemetry transmitter. This direction can be estimated analyticallyusing Maxwell's equations given the location of the source, itsorientation and the location of the magnetometer package. In the abovediscussion of use of a magnetometer (without beam steering) to detectnoise, it was assumed that for an Electric Field transmitter orientedvertically, the magnetic field will be in the horizontal plane. A moredetailed analysis reveals that the magnetic field lines arising from thetransmitter will, at the earth's surface, point along the tangent to acircle, the center of which is at the vertical projection of thetransmitter to the surface, the circumference of which passes throughthe magnetometer package, and the tangent of which is projected from themagnetometer package.

[0073] A second method of steering the three-axis magnetometer may beused when the magnetometer signals are not significantly affected bynoise correlated with the noise detected by the electric field sensor,i.e. when they detect primarily signal. In this case, the magnetometercan be steered in the direction of greatest correlation with theelectric field sensor, which will be the direction of best signaldetection. The technique for doing this is the same as the algorithmdescribed above with respect to FIG. 6. The algorithm causes thealignment of the magnetometer 98 with signal in this case because themagnetometer is detecting primarily signal. Some further noiseimprovement can be achieved by adding the signal output of FIG. 7 withthe E-field signal since random components will tend to cancel eachother.

[0074] In some cases it is possible to use one three-axis magnetometer98 and two sets of multipliers 114, each having a different set ofcoefficients α, β, and γ to provide both a signal channel and a noisechannel. This can occur when the detector is positioned so that thedetector responds primarily to signal in one direction and primarily tonoise in another. Normally this will require prior knowledge of relativelocations of the signal transmitter, the noise source and the detector.Then basic geometric calculations can be made to obtain the appropriatecoefficients α, β, and γ for signal channel and for the noise channel.

[0075] The location of the transmitter is normally known, so that it isrelatively simple to estimate the direction of signal fields. When it isknown, but the location of the noise source is not known, the noisesource direction can be measured using the method of FIG. 6 when thetransmitter is not operating.

[0076] A very simple case of selecting beam steering coefficients occurswhen the signal magnetic fields are horizontal and the noise fields arevertical. In that case, which was discussed above, the verticalmagnetometer, H_(z), would be used only for the noise channel. This isequivalent to setting the coefficients α, and β to zero for the noisechannel. Some combination of the two horizontal magnetometers wouldprovide the signal channel. This is equivalent to setting thecoefficient γ to zero and selecting appropriate values for α, and β toprovide a signal channel.

[0077] While the embodiments shown in FIGS. 6 and 7 include amagnetometer as a three-axis detector, directional E field detectors,i.e. antennas, could also be used. The antenna could be a singledirection antenna aligned with the signal or noise E field or could be athree-axis antenna. As with the three-axis magnetometer, a three-axisantenna would comprise three directional antennas positionedorthogonally to each other and would provide three outputs. The abovedescribed beam steering techniques apply to such E field detectors. Suchantennas may be particularly useful as a noise detector in offshoreapplications where it is preferred to use a magnetometer, single orthree-axis, as the signal detector. They may be useful for signaldetection in onshore applications where the signal us usually moreeasily detected as an E field.

[0078] The various magnetometer detectors discussed above provide theadvantage of a signal channel with minimum noise and/or a noise channelwith minimum signal. Such improved signal and noise channels provideimproved inputs to a noise canceller, e.g. canceller 72 of FIG. 3, andallow it to work more effectively. As discussed with reference to FIG.1, detectors 42 may preferably include motion sensors or otherelectromechanical transducers such as seismometers. Since such detectorscan be shielded so that they do not detect any EM signal, they canprovide a noise channel free of transmitted EM signal. This use ofelectromechanical transducers as EM noise channel detectors resultedfrom our discovery that hitting the side of a land drilling rig producesa response, i.e. noise, in an electrical field sensor. We believe thatthere are several mechanisms which explain why physical motioncorrelates to EM noise.

[0079] As any part of the drill rig 10 vibrates, it cuts the earth'smagnetic field lines and thus by Faraday's law (induced EMF isproportional to rate of change of magnetic flux), creates an electricfield. Where it is possible to complete an electric circuit, theelectric field creates a current, and hence another magnetic field. Anytime varying electric field creates a magnetic field and vice-versa. Thefact that a current creates a magnetic field is simply a manifestationof this same phenomenon, but is distinguished in this case because themagnetic field arising directly from the current will generally bestronger than the magnetic field arising simply from a time varyingelectric field. Hence, any vibration can be expected to correlated withelectric and magnetic noise.

[0080] Any joint between dissimilar metals will produce an electromotiveforce. As the rig 10 is stressed, the effects from joints of dissimilarmetals on the rig will vary as the contact resistance changes. Inaddition, the rig itself can act as an antenna in picking upelectromagnetic energy. Rectifying joints can demodulate high frequencyradiation, resulting in lower frequency currents having a DC componentbeing induced on the rig and acting as a noise source due to variationsin the joint as the rig is stressed by vibration.

[0081] As a drillstring is rotated in the earth's magnetic field,currents are induced in the drillstring as a consequence of Faraday'slaw (induced EMF is proportional to rate of change of magnetic flux).The amount of current will vary as the contact of the drillstring andbit with the formation varies. This serves as both a source ofelectrical and magnetic interference and can be correlated withdrillstring rotation, i.e. physical motion.

[0082] There is also some reason to expect that some of the noise willbe correlated with flowing fluids. It is well known that a streamingfluid containing clay particles creates an electromotive force.Variations in flow will thus manifest themselves as variations in theelectric field (and where it is possible for currents to flow, asvariations in the magnetic field). See e.g. P. 525 of PhysicalChemistry, Second Edition, William F. Sheehan, 1970, Allyn and Bacon,Inc., Boston. This reference also mentions another effect known as theDorn effect, which can produce a potential difference with a flowingfluid containing clay particles (e.g. drilling mud).

[0083] In addition, shale and most minerals conduct electricity. Thus,as the bit contacts the formation, an EMF is developed due to thedissimilar materials. The chemical action between the drilling mud,formation fluids and the drillstring is capable of creating anelectromotive force which can be modulated by vibration. Thus, EM noisecreated by these electrical phenomena may correlate with vibration inthe drill pipe 12.

[0084] Other types of electromechanical transducers can also provide anelectrical signal representing mechanical forces correlated with theseeffects. While a vibration detector can detect motion in the drill rig10, the motions will also cause variations in stress of the rig memberswhich can be detected by a strain gauge connected to the rig. While flowlines may produce detectable vibrations, the flow and variations in theflow can also be detected by flow rate meters and pressure detectorscoupled to the flow lines.

[0085] The noise canceling systems described above with respect to FIGS.1 through 5 each have a single reference or noise channel input. Theimproved noise detectors described herein can provide a number of noisechannels, each of which may desirably be removed from the signalchannel. FIGS. 8 and 9 illustrate systems for removing multiple noisesources.

[0086] In FIG. 8, there is shown three noise channels, labeled A, B andC. There may, of course, be more than three noise channels. Each channelis coupled through a filter 126 to an adder 128 which provides acombined noise channel to the noise channel input 130 of an adaptivefilter 132. Adaptive filter 132 may comprise the noise cancellercircuitry 72 of FIG. 3. The signal channel is coupled to primary input134 of adaptive filter 132. The signal with improved signal to noiseratio is provided on output 136.

[0087] Filters 126 preferably each include a bandpass filter to blockany frequencies outside the operating range of the EM transmitter whichgenerates the desired signal. They also preferably have transferfunctions which adjust amplitude, and possibly phase, in accordance withthe transfer function by which the various noise sources are coupled tothe signal channel detector. These adjustments to the noise channelswill help the adaptive filter 132 properly remove the noise from thesignal channel.

[0088]FIG. 9 illustrates a system in which a separate adaptive filter isused to remove each noise source from the signal channel. In FIG. 9three noise channels, A, B, and C are each coupled through filters 138to separate adaptive filters 140, 142 and 144, each of which maycomprise the noise canceller circuitry 72 of FIG. 3. In this case, thefilters 138 would provide only band pass filtering to remove frequenciesoutside the operating range of the EM transmitter which generates thedesired signal. It is not necessary to adjust amplitude and phase of thenoise channels since the adaptive filters will operate on each oneseparately.

[0089] Noise channel A is coupled to the noise channel, or reference,input 146 of adaptive filter 140. The signal channel is coupled to theprimary input 148. An improved signal from which the noise reference onnoise channel A has been removed is provided on the output 150 ofadaptive filter 140.

[0090] Noise channel B is coupled to the noise channel, or reference,input 152 of adaptive filter 142. The output 150 of adaptive filter 140is coupled to the primary input 154 of adaptive filter 142. An improvedsignal from which the noise reference on noise channel B has beenremoved is provided on the output 156 of adaptive filter 142. Sincefilter 140 has already removed noise channel A from the signal, theimproved signal on output 156 has both noise channels A and B removed.

[0091] Noise channel C is coupled to the noise channel, or reference,input 158 of adaptive filter 144. The output 156 of adaptive filter 142is coupled to the primary input 160 of adaptive filter 144. An improvedsignal from which the noise reference on noise channel C has beenremoved is provided on the output 160 of adaptive filter 144. Sinceadaptive filters 140 and 142 have already removed noise channels A and Bfrom the signal, the improved signal on output 160 has all three noisechannels A, B and C removed.

[0092] While phase shifting of the noise channels is not needed in thenormal sense, certain time delays are needed. In FIG. 9, noise channel Bis coupled through a time delay 162. This time delay is set tocompensate for the delay in the signal channel as it passes through theadaptive filter 140. This delay 162 keeps the noise channel Bsynchronized with the signal channel at the inputs 152 and 154 toadaptive filter 142. For digitized signals, this means that the delay162 may be simply a one clock cycle delay.

[0093] In similar fashion, a delay 164 is provided for noise channel C.Delay 164 is set to compensate for the time delays through both adaptivefilters 140 and 142. This delay 164 keeps the noise channel Csynchronized with the signal channel at the inputs 158 and 160 toadaptive filter 144. For digitized signals, this means that the delay164 may be simply a two clock cycle delay.

[0094] As noted above, there may be more than three noise sources havingsufficient effect on the signal channel to warrant noise cancellerapparatus. The FIG. 9 apparatus may be expanded to include a separateadaptive filter for each noise source.

[0095] In the FIG. 9 embodiment, it is preferred that the noise sourcesbe ranked in order of significance, with the most significant usuallybeing the noise having the greatest magnitude. The most significantshould be coupled to the first adaptive filter. Thus, in FIG. 9 noisechannel A would be the most significant and noise channel C would be theleast significant. This arrangement removes the biggest noise sourcefirst and should improve the efficiency of the later adaptive filterswhich will remove smaller noises.

[0096] The significance of various noise sources will not be the same atall well sites. It may also change during the drilling of a well. It istherefore preferred to use an algorithm which actively selects the bestorder in which the noise channels should be removed from the signal. Ifsignificance is based only on magnitude, the algorithm can simplymeasure amplitude of each noise channel over a period of time and rankthe noise channels by amplitude. The ranking can be done during aninitial setup of the system and, if desired, repeated on a regular basisduring drilling operations.

[0097] Not all noise channels will be of the same quality in terms ofnoise to signal ratio. The motion sensors discussed above may provide anoise channel containing essentially none of the transmitted signal.This would be a high quality noise channel because it allows a noisecanceller to remove a noise without also reducing the signal level. Evenif the magnitude of such a noise channel is less than other noisechannels, it may be considered the most significant and coupled to thefirst adaptive filter since it will have no negative effect on thedesired signal.

[0098] In similar fashion, some high amplitude noise channels may berated lower in significance for other reasons. For example, thedirectional sensors disclosed herein may provide a signal channel whicheffectively excludes some noise sources. While a noise sensor mayprovide a strong noise channel for such a noise source, there is no needto provide the channel to a noise canceller since the signal channeldoes not contain that noise.

[0099] In most cases, the noise and signal channels will be digitized asshown in FIG. 3. All processing after the digitization is normally doneby a computer programmed to perform the filtering, summing, subtracting,etc. functions. The algorithm for ranking noise channels will also beperformed by software. This allows the ranking algorithm to be performedon a real time basis and allows reordering of the noise channels on areal time basis.

[0100] It is apparent that various changes can be made in the apparatusand methods disclosed herein, without departing from the scope of theinvention as defined by the appended claims.

What we claim as our invention is:
 1. Apparatus for selectivelyreceiving electromagnetic radiation from a source of electromagneticradiation in a borehole telemetry system comprising: a directionalsensor positioned in alignment with a field generated by a source ofelectromagnetic radiation.
 2. Apparatus according to claim 1 wherein:said sensor is a magnetometer aligned with a magnetic field generated bysaid source of electromagnetic radiation.
 3. Apparatus according toclaim 1 wherein: said sensor comprises three magnetometers positionedorthogonally to each other.
 4. Apparatus according to claim 3 furthercomprising: a weighted adder for combining the outputs of said threemagnetometers to generate an output aligned with said source ofelectromagnetic radiation.
 5. Apparatus according to claim 1 wherein:said sensor is an antenna aligned with an electric field generated bysaid source of electromagnetic radiation.
 6. Apparatus according toclaim 1 wherein: said sensor comprises three antennas positionedorthogonally to each other.
 7. Apparatus according to claim 6 furthercomprising: a weighted adder for combining the outputs of said threeantennas to generate an output aligned with said source ofelectromagnetic radiation.
 8. Apparatus for transmitting electromagneticsignals from a borehole to a surface location comprising; anelectromagnetic transmitter in a borehole; and a directional sensor nearthe surface location of the borehole.
 9. Apparatus according to claim 8wherein; the directional sensor is a magnetometer aligned with thedirection of the magnetic field of signals from said electromagnetictransmitter.
 10. Apparatus according to claim 8 wherein; the directionalsensor is a set of three magnetometers positioned orthogonally to eachother.
 11. Apparatus according to claim 10 further comprising; aweighted adder for combining the outputs of said three magnetometers togenerate an output aligned with the direction of the magnetic field ofsignals from said electromagnetic transmitter.
 12. Apparatus accordingto claim 8 wherein; the directional sensor is an antenna aligned withthe direction of the electric field of signals from said electromagnetictransmitter.
 13. Apparatus according to claim 8 wherein; the directionalsensor is a set of three antennas positioned orthogonally to each other.14. Apparatus according to claim 13 further comprising; a weighted adderfor combining the outputs of said three antennas to generate an outputaligned with the direction of the electric field of signals from saidelectromagnetic transmitter.
 15. Apparatus for removing noise fromelectromagnetic signals received in a borehole electromagnetic telemetrysystem comprising; a noise sensor aligned with a source ofelectromagnetic noise.
 16. Apparatus according to claim 15 wherein: saidsensor is a magnetometer aligned with a magnetic field generated by saidsource of electromagnetic noise.
 17. Apparatus according to claim 15wherein: said sensor comprises three magnetometers positionedorthogonally to each other.
 18. Apparatus according to claim 17 furthercomprising: a weighted adder for combining the outputs of said threemagnetometers to generate an output aligned with a magnetic fieldgenerated by said source of electromagnetic noise.
 19. Apparatusaccording to claim 15 wherein: said sensor is an antenna aligned with anelectric field generated by said source of electromagnetic noise. 20.Apparatus according to claim 15 wherein: said sensor comprises threeantennas positioned orthogonally to each other.
 21. Apparatus accordingto claim 20 further comprising: a weighted adder for combining theoutputs of said three antennas to generate an output aligned with aelectric field generated by said source of electromagnetic noise. 22.Apparatus for receiving data transmitted by an electromagnetictransmitter in a borehole in the presence of a source of electromagneticnoise comprising; a directional signal sensor aligned with a fieldgenerated by said electromagnetic transmitter; a directional noisesensor aligned with a field generated by said source of electromagneticnoise; and a noise canceller having inputs coupled to said signal sensorand said noise sensor and having an output providing a signal withreduced noise content.
 23. Apparatus according to claim 22 wherein; saidsignal sensor is a magnetometer aligned with the magnetic field producedby said electromagnetic transmitter.
 24. Apparatus according to claim 23wherein: said noise sensor is a electric field sensor aligned with theelectric field produced by said source of electromagnetic noise. 25.Apparatus according to claim 23 wherein: said noise sensor is a threeaxis electric field sensor.
 26. Apparatus according to claim 25 furthercomprising: a weighted adder for combining the outputs of said threeaxis electric field sensor to generate an output aligned with a electricfield generated by said source of electromagnetic noise.
 27. Apparatusaccording to claim 22 wherein: said noise sensor is a magnetometeraligned with the magnetic field produced by said source ofelectromagnetic noise.
 28. Apparatus according to claim 27 wherein: saidsignal sensor comprises an electric field sensor aligned with theelectric field produced by said electromagnetic transmitter. 29.Apparatus according to claim 27 wherein: said signal sensor is a threeaxis electric field sensor.
 30. Apparatus according to claim 29 furthercomprising: a weighted adder for combining the outputs of said threeaxis electric field sensor to generate an output aligned with a electricfield generated by said source of electromagnetic noise.
 31. Apparatusaccording to claim 22 wherein said noise sensor and said signal sensorcomprise; three magnetometers positioned orthogonally to each other, aweighted adder for combining the outputs of said three magnetometers togenerate an output aligned with said electromagnetic transmitter; and aweighted adder for combining the outputs of said three magnetometers togenerate an output aligned with said source of electromagnetic noise.32. Apparatus according to claim 22 wherein; said noise cancellercomprises an adaptive filter.